Recently, monolithic or multi-layer ceramic capacitors have been used widely in electronic circuits of various electrical products such as, for example, electronic computors, communication devices, television receiving sets, electronic timepieces and radio receivers, because such capacitors are small in construction but large in capacity and high in reliability.
In general, the monolithic ceramic capacitors have been manufactured in the following manner: a dielectric ceramic composition is used consisting essentially of a titanate, several dielectric green sheets are prepared, and then internal electrodes are screened thereon; the green sheets are stacked and pressed to form a monolithic multi-layer body; and the resultant monolithic body is fired at a temperature of 1250.degree. to 1400.degree. C. in air to form a sintered monolithic body, and then provided at its end surfaces with terminations connected with the respective internal electrodes. Therefore, the internal electrode materials used in such a system must meet the following requirements:
(1) They must have a melting point higher than the sintering temperature of the dielectric material; and
(2) MUST NOT OXIDIZE, AND NOT REACT WITH THE DIELECTRIC MATERIAL EVEN IF HEATED UP TO ABOUT 1300.degree. C. in air.
These requirements are fully met by base or noble metals such as platinum and palladium, and such noble metals have been used with success in the past as internal electrode materials. However, these electrode materials are quite expensive so that the use of noble metals has resulted in a rise in the cost of multi-layer ceramic capacitors since they occupy about 20 to 50% of the whole cost of the capacitors.
In order to overcome this problem, various attempts have been made to use inexpensive base metals as internal electrode materials. The base metals are oxidizable and reactable with the dielectric materials under oxidizing atmospheres, thus making it impossible to provide internal electrodes on the dielectric materials. For example, if nickel is used as internal electrode materials, and heated at a temperature more than 300.degree. C. in oxidizing atmospheres, it oxidizes and reacts with the dielectric material. Accordingly, when the base metals are used as internal electrode materials, it is necessary to fire the dielectric material with electrodes in neutral or reducing atmospheres to prevent the oxidation of base metals. However, under such firing conditions, the prior art dielectric ceramic materials are reduced greatly, and the specific resistance thereof is decreased to about 10 to 10.sup.8 .OMEGA..cm, so that they cannot be used as dielectrics for capacitors.
It has been proposed to prevent the reduction of dielectric materials under the reducing atmosphere by the addition of a transition metal oxide, particularly, manganese oxide, for example, in the following publications:
(1) "High Permittivity Ceramics Sintered in Hydrogen", by J. M. Herbert, 1963;
(2) U.S. Pat. No. 3,920,781.
It is true that the addition of the transition metal oxide is effective to provide dielectric ceramic materials which are not reduced even if fired in neutral or reducing atmospheres. For example, barium titanate ceramic containing 1 mol % manganese dioxide possesses high specific resistance of approximately 10.sup.12 .OMEGA..cm even after fired in the reducing atmosphere. However, the addion of the transition metal oxide has been accompanied by such disadvantages that the curie temperature of the material is considerably affected by an amount of the transition metal oxide, a firing temperature, a firing time and a firing atmosphere, and that the aging of the insulation resistance of the material becomes extremely great, as compared with conventional dielectric ceramic compositions fired in air.
It is therefore an object of the present invention to provide a dielectric ceramic composition which possesses high permittivity and small dielectric loss, and maintains high insulation resistance even if fired in reducing atmospheres.
Another object of the present invention is to provide a dielectric ceramic composition, of which the aging is very small, and the curie temperature is scarcely affected by the firing conditions.
A further object of the present invention is to provide a dielectric ceramic composition which makes it possible to manufacture multi-layer ceramic capacitors with internal electrodes of an inexpensive unnoble metal without deterioration of dielectric properties.
According to the present invention, there is provided a non-reducing dielectric ceramic composition comprising a solid solution represented by the compositional formula: EQU {(Ba.sub.1-x Ca.sub.x)O}.sub.m.(Ti.sub.1-y Zr.sub.y)O.sub.2
wherein the subscripts m, x and y have the following values:
1.005 .ltoreq. m .ltoreq. 1.03 PA1 0.02 .ltoreq. x .ltoreq. 0.22 PA1 0 &lt; y .ltoreq. 0.20
The above dielectric ceramic composition possesses high insulation resistance even if fired in neutral or reducing atmospheres, and the electrical properties and curie temperature thereof are scarcely affected by the firing conditions. Further, the aging of the insulation resistance is extremely small, as compared with the conventional non-reducing dielectric ceramic compositions.
The non-reducing dielectric ceramic composition of the present invention is intended for practical use as a dielectric material for monolithic ceramic capacitors, thus making it possible to manufacture inexpensive but highly reliable monolithic multi-layer ceramic capacitors.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.